It is well known that the size of the primary aperture of an optical instrument limits its resolving power. In the past, efforts to increase aperture size were frustrated by the scaling problems associated with monolithic optical elements. For certain applications, such as radio astronomy, the long wave lengths of interest have allowed the use of reflectors comprised of segmented elements. Optical mirrors made from segmented optical elements can have both effectively larger apertures and lower costs than corresponding monolithic optical elements, but require actuators and control mechanisms whose precision and complexity dramatically increase with visible and short wavelength radiation applications and with the number of optical elements. An example of a segmented optical element is found in the primary aperture of the Keck ten meter telescope under construction in Hawaii, a description of which is found in the Keck Observatory Report Number 90, published by the Keck Observatory, of the California Institute of Technology, Pasedena, Ca. and, the University of California, Berkeley.
To achieve near diffraction limited performance, ground based optical systems require compensation for atmospheric propagation effects on optical ray paths. Efforts to compensate for the effects of the atmosphere on propagating optical beam wavefronts has led to the development of deformable membrane mirrors which allow an optical surface to be modulated to approximate the conjugate shape of the wavefront distortions introduced between the optic elements and the focal object, thus permitting optical quality approaching that in vacuum.
Early adaptive optics developments were directed towards ground and airborne systems using small diameter deformable and/or segmented secondary or tertiary optical elements to correct the performance of primary mirrors having diameters in the 1-2 meter range. As ground-to-space applications took on greater reality in the 1970s, the deformable mirror technology developed for small aperture systems was directed toward solving the turbulence problem for ever larger optical elements, thus necessitating scaling of the number of adaptive elements (or zones) to many hundreds or thousands.
However, existing deformable mirror technology has many drawbacks which limit upward scaling. Required sub-aperture adaptive optical element size scales inversely with beam expansion ratio. Therefore, if adaptive optical compensation does not take place in the primary beam aperture, optical element displacement actuators used in a sub-aperture (for example a tertiary) optical element must have a packing density which increases with the square of the beam expansion ratio. Such closely spaced actuators will exacerbate the already severe system cooling requirements in laser beam expanders and limit the performance of imaging systems. Moreover, existing wavefront conjugation algorithms used to position the actuators are computation intensive for large mirrors requiring many adaptive elements and require the inversion of large matrices and/or construction of expensive and complex ad hoc hardware arrays. Limited actuator stroke suitable for small deformable mirrors (less than 1 meter in diameter) is not adequate for the displacement magnitude (approximately one hundred microns) required of the conjugating surface associated with the optical path differences that will be encountered across 10 meter class primary optical apertures. In addition, optical system architectures based on either true single element monolithic primary mirrors or semimonolithic mirrors comprised of moderately large segments are almost totally inflexible with regard to design or operational changes.